The present invention relates to an FM demodulation circuit. More particularly, it relates to an FM demodulation circuit employing a phase locked loop (PLL) wherein the loop filter of the PLL is constructed as a variable loop filter, the loop band width of which is varied in correspondence with the magnitude of the carrier to noise ratio (C/N), and the degree of modulation and modulation frequency of an input signal.
A known FM demodulation circuit employing of PLL will be described with reference to FIGS. 1 to 3. Such a circuit has been used in the reception of signals from a satellite and, as shown in FIG. 1, a down link signal from a satellite is converted into a signal of an intermediate frequency (IF) in, for example, the 70 MHz band by a frequency converter circuit not shown. The intermediate frequency signal is controlled to an optimum level by an intermediate frequency amplifier circuit and an automatic gain control circuit (AGC) not shown, and is applied to a phase comparator 1 of the PLL demodulation circuit at the succeeding stage. The phase comparator 1 compares the phase of the intermediate frequency input signal and the phase of a signal from a voltage-controlled oscillator 2. An output from the phase comparator 1 generated on the basis of the difference of the phase between the two compared signals is smoothed by a loop filter 3. The output from the loop filter 3 functions as a control voltage for the voltage-controlled oscillator 2, to bring the frequency of the controlled oscillator 2 into exact agreement with the frequency of the intermediate frequency input signal. The output signal of the loop filter 3 (the control voltage to the voltage-controlled oscillator 2) is a signal obtained by demodulating an IF signal which is frequency modulated by a base band signal. After the demodulated IF or recovered base band signal is amplified by an amplifier 4, it is subjected to a deemphasis circuit 5 for adjustment by the attenuation of a higher frequency regions of the transmitted signal made by an emphasis circuit in the transmitter. A part of the corrected signal is transmitted to an audio demodulation circuit, not shown, in order to demodulate the audio signal, while the other part thereof is transmitted to a clamp circuit and a video amplifier, not shown, through a low-pass filter 6 adapted to pass the video signal band.
FIG. 2 shows the discrete circuit components of a lag-lead filter which is usually used as the loop filter 3 in FIG. 1. In FIG. 2, numerals 7 and 9 designate resistors, and numeral 8 a capacitor.
In general, factors which determine the characteristics of the PLL are the D.C. loop gain, and the loop band width of the loop filter. It is known that the D.C. loop gain is determined by the product between the conversion gain K.sub.1 of the phase comparator 1 and the voltage-to-frequency conversion gain K.sub.2 of the voltage-controlled oscillator 2. One the other hand, the loop band width of the loop filter is determined by the time constants .tau..sub.1 and .tau..sub.2 of the filter. In case of the lag-lead filter shown in FIG. 2, these time constants .tau..sub.1 and .tau..sub.2 are expressed by: ##EQU1##
Assuming here that the D.C. loop gain is constant, the characteristics of the PLL constructed of the phase comparator 1, voltage-controlled oscillator 2 and loop filter 3 (the lag-lead filter shown in FIG. 2), in other words, the characteristics of the FM demodulation circuit employing the PLL, are determined by the selection of the time constants .tau..sub.1 and .tau..sub.2 determining the loop band width. Thus, the transient response characteristics of the capture range, the noise band width, the maximum phase deviation, the maximum frequency etc. are determined. In the case of the lag-lead filter 3 in FIG. 2: ##EQU2## Since .tau..sub.1 &gt;&gt;.tau..sub.2 is selected in most cases, the natural frequency .omega..sub.n of the loop is substantially determined by the time constant .tau..sub.1 or C.sub.1 as well as R.sub.1. It can be said that the loop band width is also determined by C.sub.1 as well as R.sub.1 in this way.
FIG. 3 shows the filter characteristic of the lag-lead filter 3. It illustrates that the loop band width is substantially constant at .omega..sub.n irrespective of the modulation band width of the input signal.
Among the characteristics of the FM demodulation circuit employing the PLL, there is the tracking characteristic. In order to attain a good tracking characteristic by minimizing a transient error for the modulated wave of the input signal and reducing the jitter of an output attributed to the internal noise of the voltage-controlled oscillator 2, the state must be held in which the loop band width can be made sufficiently broad versus the required modulation band width of the input signal. In contrast, in order to reduce the jitter of the output generated by an external noise in the case where the C/N ratio of the input signal is inferior, the loop band width needs to be narrowed to the utmost. In this manner, filter constants for attaining good tracking characteristic and performing a demodulation of little distortion, and filter constants for suppressing to a small magnitude the noise of the output in the case of the inferior C/N ratio involve contrary design considerations. In the known FM demodulation circuit employing the PLL as shown in FIG. 1, both design considerations cannot be simultaneously fulfilled, and filter constants which afford the optimum condition comprehensively are selected in design. It has been impossible to simultaneously select the optimum value for each characteristic.